Low-Cycle Fatigue Characteristics of Precipitation-Hardened Superalloys at Cryogenic Temperatures

In order to examine the axial-strain-controlled low-cycle fatigue characteristics of precipitation-hardened superalloys at cryogenic temperatures, Ni-base alloy Inconel 718 and iron-base alloy A286 were selected as superalloys for cryogenic use. Low-cycle fatigue characteristics of these alloys including the weld were investigated at 300, 77, and 4 K, in order to clarify the relation between mechanical properties and fatigue strength, especially in the high-cycle fatigue life regime. As for Inconel 718, the modified alloy improving the weldability and manufacturing of large-sized ingots was used. The tests clarified that fatigue strength of both alloys increases with decreasing temperature on the basis of a relation between total strain range and fatigue life (Nf). However, even though apparent stress amplitudes are normalized by 0.2% yield strength at the same temperatures, the fatigue strength of these alloys was found to decrease with the increase of temperature in the high Nf regime. Taking 105 cycles as a typical fatigue life in the high Nf regime and adjusting the data for the relation between normalized apparent stress amplitude at 105 cycles and 0.2% yield strength, including the reference data of 300-series stainless steels, it was made clear that the higher the static strength of the alloy with stable austenite, the relatively lower the fatigue strength in the high Nf regime tends to be, when it is normalized by 0.2% yield strength at the same temperatures. Further, focusing on the alloys with stable austenite at 4 K, the relation between normalized apparent stress amplitude at 105 cycles and 0.2% yield strength was generally characterized, and an upper bound of 0.2% yield strength for providing apparent stress amplitude value at a fatigue life of 105 cycles, which is larger than 0.2% yield strength, was established.